Synchronous, or common frequency, transmission systems are well known and may be broadly defined as those which use a single carrier frequency shared by two or more transmitters that have identical program modulation, where the transmitters are located close enough to provide overlapping service areas.
It has been known, since the early days of AM broadcasting, that synchronous transmission could provide improved coverage, while not appreciably increasing interference. The system is especially attractive where dense "islands" of population are to be served. In such cases, a satellite transmitter, or transmitters, can be located close to the clusters of population in cases where they are not adequately covered by the primary or main transmitters.
The basic weakness of synchronous transmission is that it creates a zone of self interference, where signals from the primary and satellite transmitter overlap and are approximately equal in amplitude, in which carrier nulls can occur, thereby producing distortion in receivers. Such zones are called "mush zones", and it is desirable to locate them in regions of the radio stations's coverage area where there is low population density and where no major roads are located so as to minimize the number of listeners likely to encounter the distortion which results from the self interference. However, mush zones continue to be the greatest deterent to widespread use of synchronous AM transmission.
Accordingly, considerable engineering effort in the prior art has been directed toward reducing the adverse effects of self interference in the mush zones. For example, there are three basic synchronous transmission system arrangements in use.
In one form of prior art system the individual oscillators in the main and satellite transmitters, which establish the carrier frequency, operate independently and their frequencies are compared and adjusted to "zero beat" with some common standard, such as the reference signal produced by WWV. Alternatively, the frequency of the satellite oscillator is compared with that of the carrier frequency of the main transmitter. As long as the frequency difference between the main and satellite carriers is maintained accurately, say to less than one-tenth of a hertz, the mush zone is fairly narrow and well confined.
In another reform of prior art system, the main and satellite transmitter oscillators are locked in frequency and maintained in a close phase relationship. This arrangement avoids variable beating effects due to any frequency difference, but it creates, at least during the daytime under stable propagation conditions, sharp but very deep carrier cancellation nulls at specific locations in the mush zone. Accordingly, listeners that live in or close to such a null suffer poor reception. Furthermore, listeners driving through such nulls will hear significant bursts of noise and distortion. For example, when driving a car at 55 miles per hour directly along a straight line connecting the main and satellite transmitters of a synchronous station operating on a carrier frequency of 1 MHz, a listener's receiver will see a complete cycle of phase difference between the main and satellite signals about every six seconds.
Another prior art approach has been to maintain a precise frequency offset, for example .+-.0.1 Hz, between the main and satellite transmitters of a synchronous station so that the location of carrier nulls in the mush zone slowly and continuously move. Since the nulls move, they cause degradation throughout the mush zone, compared with fixed nulls which cause degradation at specific locations in the mush zone. The AVC of a typical radio receiver is able to average out these slowly moving nulls, providing a somewhat noisier signal, but one whose level is relatively constant.
My U.S. Pat. No. 4,569,073 and pending U.S. patent application Ser. No. 07/117,594, filed Nov. 5, 1987 cover assymetrical sideband AM transmission systems one of which (known as POWER-side.TM.) is presently being used experimentally for reducing the adverse effects of sideband cancellation also which occurs in the mush zone of a synchronous transmission system. The POWER-side system, which is manufactured by Kahn Communications, Inc., Westbury, N.Y., also allows listeners to favor one sideband in tuning, which in laboratory tests indicates that superior reception can be achieved under worst case conditions using this technique.
In light of the above, it is an object of the present invention to provide an improved synchronous AM transmission system wherein the adverse effect of self interference in the mush zone is reduced.